A girl that you personallyconsider attractive but you know on the inside that she is much to ugly to be seen with. This is why you date her during the summer so you will not be seen with her by your close school/work friends. This can also be used while talking about a boy.

Although Rachel had a nice figure, her face was quite ugly and she was considered undateable because of it. Jimaquanithin still found her attractive but knew he would be ruined if he was seen with her and he considered Rachel a Summer Girlfriend.